Mono- and disaccharides comprise staples in commerce with a wide variety of uses. For example, the aldohexose glucose is used as a sweetener as are the ketohexoses fructose and sorbose. Other monosaccharides, such as for example, the pentoses, arabinose, and xylose, as well as the aforementioned glucose, may be used as a major component of fermentation media. Still other monosaccharides such as mannose, may be reduced to polyols such as mannitol and sorbitol, which then find uses as humectants and excipients in tablets. Furthermore, disaccharides such as maltose also possess a degree of sweetness, especially when in pure form and not contaminated by a reversion product, may also be used as a sweetener.
One derivation of sugar is from cane sugar and sugar beets and is obtained by crushing and extracting the sugar from the cane with water or by extracting the sugar from the sugar beet with water, followed by evaporation and purification with lime absorbent carbon and/or various liquids. The chief component of this type of sugar is sucrose, the other components comprising polysaccharides. In addition, another source of glucose comprises a starch. As is well known, starch is present in many naturally occuring plants including corn, potatoes, rice, tapioca, wheat, etc.
The monosaccharides themselves generally are obtained by hydrolysis of polysaccharides, i.e., polymers having monosaccharides as their repeating units. Hydrolysis of polysaccharides sometimes is chemical, sometimes is enzymatic, and sometimes is a combination of both. For example, hydrolysis of starch, or saccharification as the process more often is called, may be a combination of acid and enzymatic (alpha-amylase) hydrolysis to afford partially hydrolyzed reaction mixture containing some mono-, di-, and trisaccharides, but composed mainly of polysaccharides with 4 or more monomeric units. Polysaccharides composed of n monomeric units often are referred to as having a "degree of polymerization n" or with the notation DPn; in this context partially hydrolyzed starch is mainly DP4, DP5, etc., collectively here designated as DP4+. The partially hydrolyzed starch, or a thinned starch as it is often called, is then further hydrolyzed enzymatically by amyloglucosidase (AG, or glucoamylase) to afford a mixture rich in monosaccharides (DP1), but also containing disaccharides (DP2), trisaccharides (DP3), and higher polysaccharides (DP4+).
In order to obtain pure mono- and disaccharides which, as hereinbefore set forth, are particularly useful as sweeteners, it is necessary to separate them from the tri-, tetra- and higher polysaccharides in an aqueous solution. Among the methods of separation which are particularly utilized to effect this separation is a membrane type separation in which the aqueous solution is passed over the surface of an ultra-filtration membrane in which the membrane possesses a molecular weight cutoff which is low enough to reject materials which possess molecular weights substantially higher than the mono- or disaccharides, but has a molecular weight cutoff high enough to permit the mono- and disaccharides to pass through the membrane at a high flux. The delicate balance between the selectivity of the membrane to reject unwanted components and a flux through the membrane of a permeate rich in mono- and disaccharides which is high enough to render the process commercially feasible has resulted in a continuing development of ultra-filtration membranes which possess the requisite characteristics.
As will hereinafter be shown in greater detail I have now discovered that an ultra-filtration membrane of the type hereinafter set forth in greater detail will permit a separation of mono- and disaccharides from higher polysaccharides whereby the permeate will be enriched in the former and the retentate will contain the latter while allowing the passage of the permeate through the membrane at a commercially feasible rate of reflux.